The poultry industry is particularly vulnerable to various diseases which plague birds, particularly those caused by respiratory viruses. One of the most enduring of such viruses is Newcastle disease virus.
Generally, the industry controls respiratory disease through vaccination, and virtually all chickens and an increasing number of turkeys produced commercially are vaccinated between one and fourteen days of age to control Newcastle disease.
Newcastle disease virus vaccine is usually combined with vaccines for other respiratory diseases of viral etiology for the convenience of the poultry producer who administers them by drinking water, eyedrop, beak-o-vac.RTM., or by spray (aerosol). The latter technique is increasing in popularity because it permits mass administration at low labor cost. The problem with this approach is an increase in the severity of Newcastle disease vaccine reactions of chickens, particularly broilers, resulting in a higher incidence of airsac disease, stunted growth, and mortality. In commercial chickens, a number of factors may exacerbate Newcastle disease vaccine reactions including infectious bursal disease virus, Marek's disease virus, and perhaps other agents that may decrease immune function and overall resistance, thus causing the attenuated vaccines to be more pathogenic for immune-compromised chickens. Hatchery-related bacterial and fungal infections are also an important consideration since they predispose baby chicks to more severe vaccine reactions.
In 1977, U.S. Pat. No. 4,053,583 described a mutagenic treatment of a parent virus strain by ultraviolet irradiation or treatment with buffered nitrous acid solution. The mutagenic treatment is pathogenic to animals (including humans) and virus alike, and inactivates most of the parent virus population. The surviving virus is isolated and cold adapted at 26.degree. C., followed by cloning at the cold adaptation temperature. Despite the claims of effectiveness for the product strain, the inventors issued U.S. Pat. No. 4,235,876 in 1980 in which they acknowledge the low infectivity of the strain produced and teach that the infectivity of at least one strain improves when the virus is treated with trypsin or passaged twice on chicken embryo fibroblasts at a non-permissive temperature of 41.degree. C. The passage of time has shown that neither the mutagenic treatment nor the modified mutagenic treatment provides an adequate answer to the problem of Newcastle disease virus vaccination.
A recent trend in the United States is to discontinue the use of inactivated adjuvant-based Newcastle disease virus vaccines in broiler breeders. Breeder hens vaccinated with inactivated vaccines confer high levels of maternal antibody to their chicks. However, Newcastle disease virus maternal antibody interferes to varying degrees with Newcastle disease virus vaccinations during the first week of life and some chicks with low Newcastle disease virus maternal antibody titers experience more severe vaccine reactions. Young broilers will suffer an increase in vaccine reactions and mortality associated with Newcastle disease virus vaccines as a result of diminished use of inactivated Newcastle disease virus vaccine.
A common response of the poultry producer to increased early vaccine reaction problems has been to cut or dilute respiratory virus vaccines. This is a risky practice, particularly when mass vaccination methods such as coarse spray or drinking water applications are used since chickens thus vaccinated are less likely to receive an adequate immunizing dose of the vaccine virus and contract the vaccine virus from a vaccinated flockmate. Bird-to-bird transmission of respiratory vaccine viruses increases virus pathogenicity and contributes to extended "rolling" reactions since chicks become infected at different times. The normal decay of maternal antibody to Newcastle disease virus and infectious bronchitis virus (IBV) also plays a role in "rolling" reactions since chicks missed during vaccination become more susceptible to vaccine reactions the later they become infected.
Consequently, an adequate Newcastle disease virus vaccine administrable to animals, including poultry, by spray (aerosol) is not yet available.